Adding and shedding loads using load levels to determine timing

ABSTRACT

Some embodiments relate to a method of adding and shedding loads that are connected to a generator. The method includes determining whether a plurality of loads is being supplied with power by the generator and then determining the total load that the generator is supplying to the plurality of loads. The method further includes determining whether to change a number of the loads in the plurality of loads based on the amount of load L that is being supplied by the generator. The method further includes determining an amount of time T in which to change the number of loads in the plurality of loads based on the amount of load that is being supplied by the generator.

TECHNICAL FIELD

Embodiments pertain to a system and method for adding and sheddingloads, and more particularly to a system and method for adding andshedding loads using load levels to determine timing.

BACKGROUND

The process of prioritizing loads that are connected to a power supplythat has limited capacity is typically referred as load shedding. As anexample, power may be supplied by a standby generator where loadshedding is required because the standby generator has a capacity thatis less than the requirements of the entire attached load.

Water heaters and air conditioners are among the commonly utilizeddevices that are powered loads by a power source (e.g., a generator).These loads may need to be shed when a residence is being supplied by alimited capacity generator. Existing load shedding systems typicallyprioritize each load and then determine if the limited capacity powersource is able to supply the loads before adding each load. If thelimited capacity power source becomes overloaded, then the load controlsystem will remove one or more loads to allow the power source tocontinue supplying power to the more important connected loads.

Utilizing a load shedding system may allow a smaller standby generatorto be installed thereby decreasing the generator costs that areassociated with powering a facility. In addition, load shedding maydecrease costs by limiting the peak demand for power during certaintimes of the day because such systems often allow a power generationutility to keep a less efficient generation plant offline and then passthe savings on to the customer (i.e., the generator user).

One of the drawbacks with existing load shedding systems is thatalthough custom-designed and configured load shedding schemes work wellunder some conditions; many load shedding systems do not work well whenoperating a variety of loads under a variety of conditions.

One of the biggest challenges for a load shedding system is ahigh-priority switching load. In one example scenario, a high-priorityswitching load may be deactivated which allows less important loads tobe added. Therefore, once the high-priority switching load is eventuallyturned on, the power source becomes overloaded. The load shedding systemmust then shed several loads before the load that is actually causingthe overload is removed. The additional time that is required to shedmultiple loads increases the likelihood of the power source becomingoverloaded for an undesirable period of time. Although many existingload shedding systems are customized in an attempt to minimizeunintended power source dropouts, such systems are still often unable toadequately handle high-priority switching loads.

Another drawback with conventional load shedding systems is that in somescenarios, all of the loads may not be drawing power from the generatorduring an overload condition. As an example, six loads may be activatedby the system even though only two of the loads are actually drawingpower. As a result, when an overload occurs after all these loads havebeen added, the system may have to take unnecessary time to shed as manyas five loads before actual load on the power source decreases at all.This increase in time to shed the appropriate load could result in thepower source going offline.

Load shedding systems must also typically be carefully configured inorder to work in each application because standard load shedding logicdoes not accurately match the load profile of a typical power source ora typical motor load. As a result, these existing systems are typicallyunable to start large motors that would otherwise typically lie withinthe starting capabilities of the generator. Configuring a typical loadshedding system to permit starting a large motor will typically resultin inadequate protection for the generator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example load shedding system.

FIG. 2 illustrates an example engine driven generator that may be usedwith the load shedding system shown in FIG. 1.

FIG. 3 illustrates an example of how time T varies when a given load isadded based on the generator load L and the available generator capacityat a point in time as compared to a conventional method of adding loads.

FIGS. 4 and 5 illustrate an example of how time T varies for a givenload being shed based on the correspond overload of a generator ascompared to a conventional method of shedding loads.

FIG. 6A shows conventional under-frequency load shedding techniqueshandling motor starting and overload conditions.

FIG. 6B shows under-frequency load shedding techniques handling motorstarting and overload conditions in accordance with some exampleembodiments.

FIG. 7 illustrates decreasing the time to shed subsequent loads after aprevious load shedding operation in accordance with some exampleembodiments.

FIG. 8 is a block diagram that illustrates a diagrammatic representationof a machine in the example form of a computer system 400 within which aset of instructions for causing the machine to perform any one or moreof the methodologies discussed herein may be executed.

DETAILED DESCRIPTION

The following description and the drawings sufficiently illustratespecific embodiments to enable those skilled in the art to practicethem. Other embodiments may incorporate structural, logical, electrical,process, and other changes. Portions and features of some embodimentsmay be included in, or substituted for, those of other embodiments.Embodiments set forth in the claims encompass all available equivalentsof those claims.

A method of adding and shedding loads L1, L2, L3, L4 that are connectedto a generator 12 will now be described with reference to FIG. 1. Themethod includes determining whether a plurality of loads L1, L2, L3, L4is being supplied with power by the generator 12 and then determiningthe total load that the generator 12 is supplying to the plurality ofloads L1, L2, L3, L4.

The method further includes determining whether to change a number ofthe loads in the plurality of loads L1, L2, L3, L4 based on the amountof load L that is being supplied by the generator 12. As shown in FIGS.3-6, the method further includes determining an amount of time T inwhich to change the number of loads in the plurality of loads based onthe amount of load L that is being supplied by the generator 12.

In some embodiments, determining whether a plurality of loads L1, L2,L3, L4 are being supplied with power by the generator 12 may includemonitoring the position of an automatic transfer switch 13. It should benoted that the plurality of loads L1, L2, L3, L4 are being supplied withpower by generator 12 when the automatic transfer switch 13 is in anemergency position.

In alternative embodiments, determining whether a plurality of loads L1,L2, L3, L4 are being supplied with power by the generator 12 may includemeasuring a position of a throttle 17 that is part of the generator 12(see e.g., FIG. 2). It should be noted that the generator 12 may beestablished as supplying power to the plurality of loads L1, L2, L3, L4when the throttle 17 position is in a position other than a “no load”position.

In still other embodiments, determining whether a plurality of loads L1,L2, L3, L4 are being supplied with power by the generator 12 may includemonitoring the generator load L. As examples, monitoring the generatorload L may be done by (i) measuring the generator 12 operatingfrequency; (ii) measuring the generator 12 operating voltage; and/or(iii) measuring the generator 12 current.

In addition, determining the total load L that the generator 12 issupplying to the plurality of loads L1, L2, L3, L4 may include (i)measuring the generator operating frequency; (ii) measuring thegenerator operating voltage; and/or (iii) measuring the generatorcurrent.

In some embodiments, determining the total load L that the generator 12is supplying to the plurality of loads L1, L2, L3, L4 includesdetermining the output torque of a prime mover (i.e., an engine) of thegenerator 12. The output torque may be calculated by (i) measuring fuelinjection time duration 18 within the generator 12; (ii) measuring theintake manifold 16 pressure within the generator 12; and/or (iii)measuring a position of a throttle 17 within the generator 12. It shouldbe noted the output torque may be calculated for spark-ignited andcompression-ignited engines as well as other types of prime movers.

Increasing the Number of Loads

In some embodiments, determining an amount of time T in which to changethe number of loads in the plurality of loads L1, L2, L3, L4 may bebased on the amount of load L that is being supplied by the generator 12includes increasing the number of loads based on an available loadcapacity of the generator 12.

As used herein, the available load capacity of the generator 12 is thedifference between the maximum loading threshold of the generator 12 anda load the generator 12 is supplying at a particular point in time. Asexamples, the maximum loading threshold of the generator may beadjustable by a user via a user interface 20 (see FIG. 1), and/or may bebased on a rating determined by a manufacturer of the generator 12. Asexamples, the user interface 20 may be part of a load control module 14,automatic transfer switch 13, generator controller 15 or a stand-alonedevice.

FIG. 3 illustrates an example of how time T varies when a given load isadded based on the generator load L and the available generator capacityat a point in time as compared to a conventional method of adding loads.The amount of time T to add a load is varied based on the availablegenerator capacity. As the available generator capacity increases, thetime T to add a load decreases.

Therefore, the method allows generator loads to be added more quicklywhen there is substantial available generator capacity and more slowlywhen there is limited available generator capacity. This time adjustmentprovides (i) improved protection to the generator as the generatorapproaches maximum capacity; and (ii) power load as quickly as possiblewhen there is minimal generator loading (as compared to conventionalmethods).

Decreasing the Number of Loads

In some embodiments, determining an amount of time T in which to changethe number of loads in the plurality of loads L1, L2, L3, L4 may bebased on the amount of load L that is being supplied by the generator 12includes decreasing the number of loads based on an overload of thegenerator 12.

As used herein, the overload of the generator 12 is a difference betweena generator load at a particular point in time and a maximum loadingthreshold of the generator. As examples, the maximum loading thresholdof the generator may be adjustable by a user interface 20 (see FIG. 1),and/or may be based on a rating determined by a manufacturer of thegenerator 12.

FIGS. 4 and 5 illustrate an example of how time T varies for a givenload being shed based on the corresponding overload of the generator 12as compared to a conventional method of shedding loads. The amount oftime T to shed a load is varied based on the overload of the generator12. As the overload increases, the time T to shed a load decreases.

Therefore, the method allows generator loads to be shed more quicklywhen there is substantial generator overload and more slowly whengenerator 12 is not as heavily overloaded. This time adjustment (i)provides improved protection to the generator 12 when there issubstantial generator overload by shedding loads more quickly (see e.g.,FIG. 4); and (ii) permits motor starting (see e.g., FIG. 5) (as comparedto conventional methods).

As shown in FIG. 6B, determining an amount of time in which to changethe number of loads in the plurality of loads based on the amount ofload that is being supplied by the generator includes decreasing thenumber of loads based on generator operating frequency. In someembodiments, the amount of time to decrease the number of loads willdecrease as the generator operating frequency decreases.

As shown in FIG. 6A, conventional under-frequency load sheddingtechniques shed load after the generator has remained below a fixedthreshold for a specified period of time. This type of operatingparameter results in poor power quality being supplied to loads andcould also result in unintended shedding during motor starting,especially when using heavily loaded large AC motors.

Comparing FIGS. 6A and 6B demonstrates how the methods described hereinmay improve on conventional under frequency load shedding techniques.FIG. 6A illustrates conventional under frequency load sheddingtechniques for a given motor starting load and a given overload whileFIG. 6B illustrates the under frequency load shedding techniquesdescribed herein for the same motor starting load and the same overload.

It should be noted that while FIGS. 3, 4, 5 and 6 illustrate lineartime/load curves, other embodiments are contemplated where these curvesmay be non-linear. The shape of these curves will depend on a variety ofdesign considerations.

FIGS. 1 and 7 illustrate a method of adding and shedding loads that areconnected to a generator in accordance with another example embodiment.The method includes determining whether a plurality of loads L1, L2, L3,L4 is being supplied with power by the generator 12 and determining theload L that the generator is supplying to the plurality of loads L1, L2,L3, L4.

The method further includes determining whether to change a number ofthe loads in the plurality of loads L1, L2, L3, L4 based on the amountof load that is being supplied by the generator 12 and changing thenumber of loads in the plurality of loads L1, L2, L3, L4. The methodfurther includes determining an amount of time in which to furtherchange the number of loads where the amount of time is determined bywhether the number of loads increases or decreases during the previouschange of the number of loads.

In some embodiments, determining an amount of time in which to furtherchange the number of loads in the plurality of loads L1, L2, L3, L4includes increasing the amount of time to decrease the number of loadswhen the previous change of the number of loads increased the number ofloads.

Other embodiments are contemplated where determining an amount of timein which to further change the number of loads in the plurality of loadsincludes decreasing the amount of time to decrease the number of loadswhen the previous change of the number of loads decreased the number ofloads.

It should be noted that embodiments are also contemplated wheredetermining an amount of time in which to further change the number ofloads in the plurality of loads L1, L2, L3, L4 includes decreasing theamount of time to decrease the number of loads when the previous changeof the number of loads decreased the number of loads.

In still other embodiments, determining an amount of time in which tofurther change the number of loads in the plurality of loads L1, L2, L3,L4 includes increasing the amount of time to increase the number ofloads when the previous change of the number of loads decreased thenumber of loads.

FIG. 7 illustrates decreasing the time to shed subsequent loads after aprevious load shedding operation. In the example scenario that isillustrated in FIG. 7, three of six loads are not demanding power fromthe generator which results in no decrease to the generator load whenthese loads are shed. The subsequent decreases in the time to shed eachload allows these loads to be shed before there is significantdegradation to the quality of power being supplied to these loads.

The methods described herein may permit load control operation that workwell when there a variety of loads that operate under a variety ofconditions. In addition, the methods may be able to more adequatelyhandle high-priority switching loads. The methods may also reduce thetime to shed multiple loads more quickly until the actual load on thepower source decreases. This decrease in time to shed the appropriateload may allow the power source to remain online.

Example Machine Architecture

FIG. 8 is a block diagram that illustrates a diagrammatic representationof a machine in the example form of a computer system 400 within which aset of instructions for causing the machine to perform any one or moreof the methodologies discussed herein may be executed. In someembodiments, the computer system 400 may operate in the capacity of aserver or a client machine in a server-client network environment, or asa peer machine in a peer-to-peer (or distributed) network environment.

The computer system 400 may be a server computer, a client computer, apersonal computer (PC), a tablet PC, a set-top box (STB), a PersonalDigital Assistant (PDA), a cellular telephone, a Web appliance, anetwork router, switch or bridge, or any machine capable of executing aset of instructions (sequential or otherwise) that specify actions to betaken by that machine. Further, while only a single machine isillustrated, the term “machine” shall also be taken to include anycollection of machines that individually or jointly execute a set (ormultiple sets) of instructions to perform any one or more of themethodologies discussed herein.

The example computer system 400 may include a processor 460 (e.g., acentral processing unit (CPU), a graphics processing unit (GPU) orboth), a main memory 470 and a static memory 480, all of whichcommunicate with each other via a bus 408. The computer system 400 mayfurther include a video display unit 410 (e.g., liquid crystal displays(LCD) or cathode ray tube (CRT)). The computer system 400 also mayinclude an alphanumeric input device 420 (e.g., a keyboard), a cursorcontrol device 430 (e.g., a mouse), a disk drive unit 440, a signalgeneration device 450 (e.g., a speaker), and a network interface device490.

The disk drive unit 440 may include a machine-readable medium 422 onwhich is stored one or more sets of instructions (e.g., software 424)embodying any one or more of the methodologies or functions describedherein. The software 424 may also reside, completely or at leastpartially, within the main memory 470 and/or within the processor 460during execution thereof by the computer system 400, the main memory 470and the processor 460 also constituting machine-readable media. Itshould be noted that the software 424 may further be transmitted orreceived over a network (e.g., network 380 in FIG. 8) via the networkinterface device 490.

While the machine-readable medium 422 is shown in an example embodimentto be a single medium, the term “machine-readable medium” should betaken to include a single medium or multiple media (e.g., a centralizedor distributed database, and/or associated caches and servers) thatstore the one or more sets of instructions. The term “machine-readablemedium” shall also be taken to include any medium that is capable ofstoring, encoding or carrying a set of instructions for execution by themachine and that cause the machine to perform any one or more of exampleembodiments described herein. The term “machine-readable medium” shallaccordingly be taken to include, but not be limited to, solid-statememories and optical and magnetic media.

Thus, a computerized method and system are described herein. Althoughthe present invention has been described with reference to specificexample embodiments, it will be evident that various modifications andchanges may be made to these embodiments without departing from thebroader spirit and scope of the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative ratherthan a restrictive sense.

The Abstract is provided to comply with 37 C.F.R. Section 1.72(b)requiring an abstract that will allow the reader to ascertain the natureand gist of the technical disclosure. It is submitted with theunderstanding that it will not be used to limit or interpret the scopeor meaning of the claims. The following claims are hereby incorporatedinto the detailed description, with each claim standing on its own as aseparate embodiment.

What is claimed is:
 1. A method of adding and shedding loads that areconnected to a generator, the method comprising: determining whether aplurality of loads is being supplied with power by the generator;determining the load that the generator is supplying to the plurality ofloads; determining whether to change a number of the loads in theplurality of loads based on the amount of load that is being supplied bythe generator; determining an amount of time in which to change thenumber of loads in the plurality of loads relative to the amount of loadthat is being supplied by the generator, wherein the amount of time tochange the number of loads varies relative to an available load capacityfor the generator for all possible loads within a particular load rangeof loads, and wherein determining an amount of time in which to furtherchange the number of loads in the plurality of loads includes increasingthe amount of time to increase the number of loads in the plurality ofloads when a previous change of the number of loads decreased the numberof loads; and changing the number of loads based on the determinedamount of time.
 2. The method of claim 1, wherein determining whether aplurality of loads are being supplied with power by the generatorincludes monitoring the generator load.
 3. The method of claim 2,wherein monitoring the generator load includes measuring the generatoroperating frequency, generator operating voltage, or generator operatingcurrent.
 4. The method of claim 2, wherein monitoring the generator loadincludes monitoring the torque of a prime mover within the generator. 5.The method of claim 4, wherein monitoring the torque of a prime moverwithin the generator includes measuring fuel injection time durationwithin the generator.
 6. The method of claim 4, wherein monitoring thetorque of a prime mover within the generator includes measuring theintake manifold pressure within the generator.
 7. The method of claim 4,wherein monitoring the torque of a prime mover within the generatorincludes measuring a position of a throttle within the generator.
 8. Themethod of claim 1, wherein determining the load that the generator issupplying to the plurality of loads includes measuring the current thatthe generator is supplying to the plurality of loads.
 9. The method ofclaim 1, wherein determining the load that the generator is supplying tothe plurality of loads includes measuring the generator operatingfrequency.
 10. The method of claim 1, wherein determining the load thatthe generator is supplying to the plurality of loads includes measuringthe generator operating voltage.
 11. The method of claim 1, determiningthe load that the generator is supplying to the plurality of loadsincludes monitoring the torque of a prime mover within the generator.12. The method of claim 11, wherein monitoring the torque of a primemover within the generator includes measuring fuel injection timeduration within the generator.
 13. The method of claim 11, whereinmonitoring the torque of a prime mover within the generator includesmeasuring the intake manifold pressure within the generator.
 14. Themethod of claim 11, wherein monitoring the torque of a prime moverwithin the generator includes measuring a position of a throttle withinthe generator.
 15. The method of claim 1, wherein determining an amountof time in which to change the number of loads in the plurality of loadsbased on the amount of load that is being supplied by the generatorincludes increasing the number of loads based on an available loadcapacity of the generator.
 16. The method of claim 15, wherein theavailable load capacity of the generator is a difference between amaximum loading threshold of the generator and a generator load at aparticular point in time.
 17. The method of claim 16, wherein themaximum loading threshold of the generator is adjustable by a user. 18.The method of claim 16, wherein the maximum loading threshold of thegenerator is based on a rating provided by a manufacturer of thegenerator.
 19. The method of claim 15, wherein the amount of time toincrease the number of loads will increase as the available loadcapacity decreases.
 20. The method of claim 1, wherein determining anamount of time in which to change the number of loads in the pluralityof loads based on the amount of load that is being supplied by thegenerator includes decreasing the number of loads based on an overloadof the generator.
 21. The method of claim 20, wherein the overload ofthe generator is a difference between a generator load at a particularpoint in time and a maximum loading threshold of the generator.
 22. Themethod of claim 21, wherein the maximum loading threshold of thegenerator is adjustable by a user.
 23. The method of claim 21, whereinthe maximum loading threshold of the generator is based on a ratingprovided by a manufacturer of the generator.
 24. The method of claim 21,wherein the amount of time to decrease the number of loads will decreaseas the generator overload increases.
 25. The method of claim 1, whereindetermining an amount of time in which to change the number of loads inthe plurality of loads based on the amount of load that is beingsupplied by the generator includes decreasing the number of loads basedon generator operating frequency.
 26. The method of claim 25, whereinthe amount of time to decrease the number of loads will decrease as thegenerator operating frequency decreases.
 27. A method of adding andshedding loads that are connected to a generator, the method comprising:determining whether a plurality of loads is being supplied with power bythe generator; determining the load that the generator is supplying tothe plurality of loads; determining whether to change a number of theloads in the plurality of loads based on the amount of load is beingsupplied by the generator; changing the number of loads in the pluralityof loads; determining an amount of time in which to further change thenumber of loads in the plurality of loads, wherein the amount of time isdetermined by whether the number of loads increases or decreases duringthe previous change of the number of loads, wherein determining anamount of time in which to further change the number of loads in theplurality of loads includes increasing the amount of time to increasethe number of loads in the plurality of loads when the previous changeof the number of loads decreased the number of loads; and changing thenumber of loads based on the determined amount of time.
 28. The methodof claim 27, wherein determining an amount of time in which to furtherchange the number of loads in the plurality of loads includes increasingthe amount of time to decrease the number of loads in the plurality ofloads when the previous change of the number of loads increased thenumber of loads.
 29. The method of claim 27, wherein determining anamount of time in which to further change the number of loads in theplurality of loads includes decreasing the amount of time to increasethe number of loads in the plurality of loads when the previous changeof the number of loads increased the number of loads.
 30. The method ofclaim 27, wherein determining an amount of time in which to furtherchange the number of loads in the plurality of loads includes decreasingthe amount of time to decrease the number of loads in the plurality ofloads when the previous change of the number of loads decreased thenumber of loads.